The scrubbing of stack gases and process gases to cleanse them of gaseous pollutants--mainly sulfur dioxide but in some cases phosphorous pentoxide--has heretofore been expensive both in operating costs and in capital investment for scrubbing apparatus. However, scrubbing is mandatory where gas to be emitted into the atmosphere contains more than specified negligible quantities of such pollutants.
In view of the increasing cost of energy and the need to conserve existing energy resources, a major consideration in evalulating the operating cost of any gas scrubbing system is its energy economy. But it is also of great importance that the waste material resulting from gas scrubbing be kept to a minimum because such material has little or no commercial value and in many cases substantial cost is entailed in disposing of it. It is also very important that the waste material from a gas scrubbing system be such as will not be a source of pollution in itself.
In one prior system for removal of sulfur from stack and process gases, now in widespread use, the gas to be scrubbed is passed through a slurry consisting of water and comminuted limestone (calcium carbonate) or a substance containing a substantial percentage of calcium carbonate. Although such a wet scrubbing system is satisfactorily effective in removing sulfur dioxide from gas to be scrubbed it has several very important disadvantages. A substantial amount of power is needed to force the gas through the slurry, and there is a further loss of otherwise available energy in cooling of the gas to be scrubbed as it passes through the slurry. The waste material from such a system is in the form of a wet sludge that must be dumped in an area where it can drain without causing possible pollution of ground waters and surface streams. Of course a wet-scrubbing system requires an adequate water supply, which may not be available in arid or semi-arid locations. Apparatus needed for wet scrubbing is high in first cost because it must be impervious to the sulfuric acid generated in the course of the adsorption reaction, and it is expensive to maintain because of a tendency for the wet material to form crusts and scales on the apparatus and to plug it from time to time. Constant skilled supervision is necessary; hence, there is a high operating cost for labor. Although the reliability of wet scrubbing systems has improved in the course of time, such systems are regarded as being, at best, somewhat unreliable and subject to unsatisfactorily frequent shut-down for repairs and maintenance.
In another prior type of gas scrubbing system, not yet in substantial commercial use, ground and hydrated (slaked) lime is mixed with water under heat and pressure to form a calcium hydroxide slurry that is sprayed into the gas to be scrubbed. The waste product of a spray system, being dry, is not as objectionable as that from a wet scrubber. But the hydrated lime that the system requires must be of relatively high quality. Furthermore, preparation of the hydrated lime entails the expenditure of a substantial amount of energy which cannot in any practical way be recovered for employment in the scrubbing operation and which should therefore be counted as part of the total energy cost of the system. In addition to being costly to operate and low in overall energy efficiency, a spray system requires complicated and costly apparatus and will probably be found to need close and constant supervision because it presents the possibility of crusting, plugging and corrosion.
Certain dry scrubbing processes have been proposed that employ catalytic materials. All such systems appear to require very complicated equipment that involves high capital and operating costs, and none of them has gone into substantial commercial use.
A gas scrubbing process has also been proposed that would employ dry sodium bicarbonate (so-called nahcolite) as the sulfur dioxide adsorbing medium. While that system is promising in theory, natural nahcolite is found only in limited geographical areas, where it lies in beds which have not yet been mined and which it is not feasible to exploit under present conditions.
Such other proposals as have come forward have at best provided only relatively minor reductions in the cost and difficulty of gas scrubbing. Efforts to solve the problem have been diligent and persistent. The urgency of the problem is well known because the cost of pollution control is widely recognized as a heavy but unavoidable burden upon the economy that yields no direct gain in productivity. It is evident, therefore, that any substantial breakthrough must result from something well beyond the application of mere skill in the art.
The present invention constitutes such a breakthrough. Its general object is to provide for scrubbing of stack and process gases with the use of a plentiful and inexpensive adsorbing medium by a process which consumes relatively little energy, can be practiced with simple, inexpensive and reliable apparatus and with a minimum of operating personnel, and produces a dry, non-polluting spent material.
It is also an object of this invention to provide a highly efficient but inexpensive and reliable method and apparatus for removing gaseous pollutants--such as sulfur and phosphorous oxides--from stack and process gases, whereby the pollutant content of such gases can be reduced from very high levels, on the order of thousands of parts per million, to very satisfactorily low levels, without incurring incidental pollution problems such as the addition of particulates to the discharged gases or the creation of a spent product that may become a source of water pollution.
Thus the principal object of the present invention is to provide a gas scrubbing system which as compared with prior commercially feasible gas scrubbing systems, has the several important advantages of: Lower capital cost, substantially lower energy consumption, capability for operating with less supervision and with much greater reliability, use of a readily available adsorbing medium, and production of a non-polluting spent product.
A more specific object of the invention is the provision of a gas scrubbing system that can operate advantageously with the employment of kerogen-bearing shale as a sulfur dioxide adsorbing medium, even through such shale has a high sulfur content contributed by both the kerogen and pyrite.
It is also a specific object of the invention to provide an efficient process and apparatus by which titanium oxide can be removed from gases to be emitted into the atmosphere, to permit recovery of its titanium value.